IAL 14: Mars: the Red Planet

Don't Panic

Sections

Introduction
Mars Facts
Mars Geography
Mars Geology, Except for Water Geology
The Martian Atmosphere
Water on Mars
Life on Mars

Introduction

Mars is the most fascinating---in general opinion---and sometimes the most ominous of the unearthly planets.

See the image of full Mars in the figure adjacent/below (local link / general link: mars_full.html).

Mars in Myth and Early History:
Mars has been known since prehistory, of course---it's one of the historical planets (i.e., Mercury ☿, Venus ♀, Mars ♂, Jupiter ♃, Saturn ♄).
The ancient Babylonians associated Mars with their war and pestilence god Nergal. The ancient Greeks with their war god Ares---of whom few amusing stories are told.
Mars is, of course, also associated with the Roman war god Mars who was identified with Greek war god Ares.
Mars has always been a favorite with astrologers---Mars in your sign is pretty easily interpreted by the Art. With artists less so---they've generally preferred Venus---but Botticelli managed to combine the twain. See the figure below (local link / general link: botticelli_venus_and_mars.html).
Mars has a memorable symbol: ♂. See the figure below (local link / general link: alien_mars_symbol.html).
Mars as Scientific Object: The Beginning:
As a scientific object, Mars came to the fore in the early 17th century when Johannes Kepler's (1571--1630) study of the orbit of Mars led him to discover that the planets had elliptical orbits. Note Martian orbit has a relatively high eccentricity of 0.0934 = 9.34 %, and so is a good case for the discovery of elliptical orbits. Among the historical planets (i.e., Mercury ☿, Venus ♀, Mars ♂, Jupiter ♃, Saturn ♄) only Mercury has a higher eccentricity (see file table_solar_system_planets.html), but Kepler may NOT have had such good data for it.
The nature of Mars itself began to be elucidated by the telescope. Using a telescope, Christian Huygens (1629--1695) identified the largest dark region on Mars, Syrtis Major.
In the 18th century, it was discovered that Mars has polar caps and sooner or later that they decrease in the summer hemisphere and increase in the winter hemisphere ???.
In the 19th century???, it was noted that the dark areas on Mars also seemed to increase in springtime suggesting increasing vegetation.
Alas, this effect is really due to spring??? winds blowing lighter colored Martian dust off darker surfaces (HI-183).
Maps of Mars were produced in the 19th century, but really how good they were by modern standards, yours truly doesn't know---probably so-so for the largest features and rotten for anything smaller. See Giovanni Schiaparelli (1835--1910) map of Mars in the figure below.
The Martian Canals:
In 1877 during a FAVORABLE opposition of Mars, Italian astronomer Giovanni Schiaparelli (1835--1910) concluded the Martian canals were a major feature (Ka-293). The canals were apparent linear markings on Mars.
Actually in calling them canali (singular canale) in Italian, Schiaparelli was NOT necessarily implying that they were artificial: his meaning may have been probably closer to the English channels---but Schiaparelli made no effort to correct the English translation---he may NOT have noticed it much (Wikipedia: Martian canals: History of canals).
Canale does have canal as its primary meaning in Italian.
Two images in the figure below (local link / general link: martian_canals.html) show the Martian canals in the imagination of the artist.
The Martians:
The Martian canals they became in popular conception and some people became convinced they were constructions of intelligent beings: the Martians.
The most prominent of the believers in Martians was Percival Lowell (1855--1916)---who founded the Lowell Observatory in Flagstaff, Arizona. See Lowell in the figure below (local link / general link: percival_lowell.html).
Lowell did a lot of Mars observations by himself. Three of his less elaborate drawings of Mars are shown in the figure below (local link / general link: percival_lowell_mars_map.html).
Because of the popular interest in
Martians, they invaded fiction---most famously in H. G. Wells' (1866--1946) The War of the Worlds (1898)---see the two figures below (local link / general link: h_g_wells.html; local link / general link: h_g_wells_war_of_the_worlds.html).
Also famously,
Orson Welles (1915--1985) presented a radio play version of The War of the Worlds (1898) in 1938--- for young Orson and an explication of the Martian invasion scare, see the figure below (local link / general link: orson_welles.html).
The Truth About Martians:
Many/most astronomers NEVER believed in the Martian canals and always thought they were optical illusions---and all the talk about Martians was vastly premature.
However, even up to the 1950s, Martians and Martian canals were considered plausible at least in scifi and in scifi illustrations---like those of Chesley Bonestell (1888---1986) and in the figure below (local link / general link: alien_prototype_martian.html).
But the final
nail in the Martians' coffin was provided by the images from the Mariner 4 flyby of Mars in 1965.
The Mariner 4 images and other work showed Mars was a cratered world with a very low density atmosphere.
It was a quite disappointment to young scifi fans like me in the 1960s. No Martian canals, no Martians, no Thuvia, Maid of Mars. See the figure below (local link / general link: thuvia_maid_of_mars.html).
Nowadays, we see
Mars as a world intermediate between the large rocky planet Earth and small, airless rocky bodies like the Moon.
But mysteries remain:
1. Mars has water today in vapor and solid form and clearly it has had liquid water in the past, but when and how much?
  Is there sub-surface liquid water today?
2. The Mars ocean hypothesis---did Mars ever have an ocean? The hypothesis is that a vast ocean covering about a third of Mars existed in the northern hemisphere. It's NOT established or dismissed.
3. Was there ever life on Mars in an earlier warmer, wetter time?
  Is there life today below the surface in a deep biosphere: perhaps like the Earth's deep biosphere.
  We've know since circa 1990 that there is a deep biosphere on Earth and microbial life can be found down to several kilometers in the crust (HI-196).

Mars Facts

"Just the facts, ma'am."

---

Joe Friday

Just some basic Mars facts---which, of course, beg for an explanation---and there is some explanation, but also some "Just So."

A Mars Fact Table:

Below we give a table of some Mars facts.

________________________________________________________________________________________________

Table:  Mars Facts
________________________________________________________________________________________________

Quantity                            Value

________________________________________________________________________________________________

Mean distance from the Sun    1.5237 astronomical units

Eccentricity of orbit         0.0934 :   which is the much larger than 
                                         Earth's 0.0167
                                         but much smaller than 
                                         Mercury's 0.206.

Mean inclination to Ecliptic  1.85061° 

Axis tilt to orbital pole     25.19°

Orbital period                686.95 days = 1.88071105 years

Rotational Period             1.02595675 days : This is the sidereal period

Martian day                   1.02749 days : just a bit longer than 
                                             the rotational period 
                                             for the same that Earth's day 
                                             is longer than its rotational period.

Equatorial radius             3,397 km = 0.53 R_Earth_equatorial

Mass                          6.424*10**23 kg = 0.1075 Earth masses

Mean density                  3.94 g/cm**3 : significantly less than
                                              mean Earth density

Uncompressed mean density     3.3 g/cm**3 : significantly less than
                                             uncompressed mean Earth density 
                                             and about same as the Moon's 
                                             uncompressed and compressed mean
                                             density.

Surface gravity               0.379 Earth gravities

Surface temperature           about -80 C (190 K) at night to -30 C (240 K)
                                in the day on a summer day.  This is 
                                surface air temperature.  On a hot day
                                in summer the ground temperature can rise
                                above freezing (HI-187).

Moons                          The Martian moons are
                               Phobos with mean radius 11.1 km
                               and
                               Deimos with mean radius 6.2 km.
                               They are both irregular in shape and 
                               may be captured asteroids,
                               but there are rival theories.

________________________________________________________________________________________________

Sources: Cox-294,295; Se-418,479; HI-187.

________________________________________________________________________________________________

The Martian Moons:
The Martian moons are two: Phobos (mean radius 11.2667 km) and Deimos (mean radius 6.2 ± 0.18 km).
They were discovered by Asaph Hall (1829--1907) in 1877 during a favorable Mars opposition---the same one that led old Giovanni Schiaparelli (1835--1910) to concluded that the Martian canals were a major feature of Mars.
The most common theory is that the Martian moons are captured asteroids, but rival theories exist (see Wikipedia: Moons of Mars: Origin).
Both Martian moons are tidally locked to Mars, and so turn the same face to Mars all the time (see Wikipedia: Moons of Mars: Characteristics).
The tidal force on Phobos is causing it to inspiral and it will break up and impact in fragments on Mars probably in 30--50 Myr (see Wikipedia: Phobos: Predicted destruction). On the other hand, the tidal force on Deimos is causing it to outspiral. See Wikipedia: Moons of Mars: Characteristics and Caleb A. Scharf, 2015, SciAm.
For the Martian moons, see the two figures below (local link / general link: martian_moons_phobos.html; local link / general link: martian_moons_to_scale.html).
We don't have more to say on the Martian moons.
The Density, Iron Core, and Magnetic Field of Mars:
Oddly the uncompressed density of Mars is similar to that of the Moon.
This suggests that Mars doesn't have a large iron core relative to Earth???.
A molten iron core is probably necessary for a large dipole magnetic field. Electrical currents generated in the iron core by convection and rotation generate the dipole magnetic field---somehow he said while handwaving.
Since Mars has no large dipole magnetic field now, its iron core is NOT sufficiently large and/or sufficiently molten.
The Earth's magnetic field and the magnetic field of Mars are illustrated in the figure below.
The Earth's magnetic field protects the Earth's atmosphere from the blasting effect of the solar wind: i.e., solar wind sputtering atmospheric escape The solar wind tends to blow away an exposed atmosphere.
The lack of strong global magnetic field means that the Martian atmosphere is directly exposed to the solar wind (see Wikipedia: Martian atmosphere: History). This seems to be a major factor Mars evolving to have a very low the density atmosphere compared to Earth's atmosphere. The average atmosphere pressure on the Martian surface is about 0.006 that of the average atmosphere pressure at Earth's sea level. Roughly speaking the Martian atmosphere is 100 times thinner than the Earth's atmosphere.
However, the low gravity of Mars (about 0.4 of Earth) and thermal atmospheric escape is probably the main reason for the relatively thin Martian atmosphere.
For more on atmospheric escape, see the figure below (local link / general link: solar_system_atmospheric_escape.html).

Group Activity:

Form groups of 2 or 3---NOT more---and tackle Homework 14 problems 2--7 on Mars.

Discuss each problem and come to a group answer.

Oh, 5--10 minutes.

See Solutions 14.

The winners get chocolates.

Mars Geography

All Mars Maps:
As a first glance at Martian geography, see the two Mars maps in the figure below (local link / general link: nasa_mars_map_000.html).
The Western Hemisphere of Mars:
The western hemisphere of Mars is shown the figure below (local link / general link: mars_full.html).
Valles Marineris is compared to the United States in the figure below.
A Topographic Map of Mars:
The collage-image map (see figure above: local link / general link: nasa_mars_map_000.html) and western hemisphere image (see figure above: local link / general link: mars_full.html) do NOT directly reveal the strong distinctions between the northern and southern hemispheres of Mars, except for more cratering in the southern one.
A topographic map reveals the elevation distinction. See the figure below.

Mars Geology, Except for Water Geology

Nowadays there's just too much known about the Martian geology.

Too much for us.

So we'll skim.

Mars in the Middle:
Because of its intermediate size between Earth and the Moon, we expect Mars to be intermediate in geological drivers.
Recall the bigger the rocky body, the more primordial-radiogenic heat geology it has (see also Wikipedia: Earth's internal heat budget: Radiogenic heat: Primordial heat).
The upshot is that Earth and Venus have a lot of primordial-radiogenic heat geology. The Moon virtually none.
Mars is in the middle in size---about 1/10 of the Earth in mass, but about 8 times the Moon. So Mars has some primordial-radiogenic heat geology: much less than Earth and Venus, but a lot more than the Moon.
Also like Earth and Venus, Mars' primordial-radiogenic heat geology has been slowing down since the Formation of the Solar System 4.6 Gyr ago.
The Geological Evolution of Mars:
Like Earth and the Moon, just after formation, Mars 4.6 Gyr ago went through chemical differentiation and cratering under the heavy bombardment and probably had massive lava flows in giant basins---any trace of this on Earth has been erased if it ever happened. Then continuing geological activity.
The 4 stages of evolution just mentioned (Se-427) are summarized in the cartoon in the figure below (local link / general link: rocky_body_evolution_4_stages.html).
On the
Earth, the impact craters of the heavy bombardment have been erased; on the Moon, many of them remain except where erased by subsequent impactors (leaving newer impact craters) and by the formation of the maria.
Mars retains a lot of the impact craters, probably many??? from the heavy bombardment (circa 4.6--3.8 Gyr ago) in the southern hemisphere. See the topographic map of Mars below.
Hellas Planitia in the southern hemisphere may be a remnant of the early lava flooding of an impact basin (HI-185). So it may be a sort of a lunar mare---but on Mars.
The northern hemisphere is much less cratered and probably impact craters there are from after the heavy bombardment, but still relatively long ago. The northern hemisphere seems to have been resurfaced by lava flows.
We discuss
water on Mars in section Water on Mars, but here we can say there is water on Mars has been more in the past, probably intermittantly, but that in any case erosion by water has been far less than on Earth.
So water erosion has also been much less important on Mars than on Earth. This is another reason for the surface of to be generally much older than that of Earth, but still less old than that of the Moon.
Also, it was once thought (circa 2005) that primordial-radiogenic heat geology was actively melting water ice periodically to the present day. But this is now doubtful. There may have been no flowing liquid water on Mars for a long time. Nothing is settled on this issue. See the section Water on Mars below.
Recall Mars' primordial-radiogenic heat geology has been slowing down since the Formation of the Solar System 4.6 Gyr ago.
So Mars' volcanism has been slowing down, but unlike that of the Moon and other small rocky body without tidal-flexing-heating geology, Mars still has slightly active volcanoes it is thought. We take up the subject of Mars' volcanism in the next subsection.
Volcanoes on Mars:
Mars has volcanoes most notably the four big ones of the Tharsis region.
The biggest of the four and biggest known in the Solar System, in fact, is Olympus Mons---see the figure below.
Olympus Mons is a shield volcano. See cartoon of a shield volcano in the figure below (local link / general link: volcano_shield_cartoon.html).
Recall
shield volcanoes are formed by repeated lava flows and have gentle slopes (SWT-572)---as illustrated in the diagram to the right.
Shield volcanoes occur on Venus, Earth, and Mars.
Another image of Olympus Mons in shown in the figure below.
See the video Vuela sobre el monte Olympus de Marte | 0:1:55 (an animation of a flight over Olympus Mons) in Mars videos below (local link / general link: mars_videos.html):
Mars Lives---Geologically Speaking:
It was once thought that Mars's primordial-radiogenic heat geology was dead, like the Moon's---but only more recently deceased.
But now there are lava flows judged to be less than 100 Myr old from crater counts (HI-189).
Less than 100 Myr is almost nothing on the time-scale of internal heat loss for a Mars-size body.
Thus, Mars has primordial-radiogenic heat geology and volcanism today---although at a much lower level of activity than Earth or Venus.
Of course, we may have to wait millennia or millions of year for the next volcanic eruption.
Plate Tectonics on Mars?
Does Mars have plate tectonics?
Apparently, NOT.
There is no solid evidence for divergent boundaries or convergent boundaries.
It is theorized that Valles Marineris (see below local link: mars_videos.html) is large rift valley where the Martian crust is being pulled apart (see Wikipedia: Valles Marineris: Formation). On Earth, rift valleys can be divergent boundaries between tectonic plates (see Wikipedia: Rift valley), but Valles Marineris does NOT seem to fall into that class.
Valles Marineris may be plate-tectonics feature of Martian plate tectonics that never developed very far.
There are no real continent or terra formations. (Terras are the continent-like features on Venus.)
Volcanism on Mars may be all hotspot volcanism. The Tharsis region may have been formed by a giant hotspot and may be similar to the coronas on Venus (HI-191).
See Valles Marineris in Mars videos below (local link / general link: mars_videos.html):
The Future of Mars:
In the future, of course, primordial-radiogenic heat geology must slow on Mars as primordial heat from formation and past radioactive decay decreases. It must turn off eventually.
But probably this won't happen until long after the Sun becomes a white dwarf.
Mars probably won't be vaporized in the Sun's red giant and AGB phases and may continue to orbit the white dwarf Sun in some orbit for hundreds of gigayears into the future????.
The orbit of Mars will certainly be different in that far future since the Sun will lose a lot of mass in becoming a white dwarf, and thus its gravitational force will be weakened. Probably the orbit will be much larger.

The Martian Atmosphere

The Martian atmosphere is thin.

The Thinness of the Martian Atmosphere:
The Martian atmosphere is shown in the figure below.
The surface pressure that is about 1 % of Earth's.
The Existence of Liquid Water on Mars:
The low pressure of the Martian atmosphere means that liquid water CANNOT exist on the surface for any length of time.
Under low enough pressure, the liquid phase doesn't occur for most (all?) substances: they condense from gas to solid and vaporize (in loose astro jargon) from solid to gas.
The figure below (local link / general link: co2_ice.html) gives an example of a substance that under standard temperature and pressure (STP) has NO liquid phase.
Water below 0.01 atmospheres does NOT have a liquid phase. See the phase diagram of water in the figure below (local link / general link: phase_diagram_water.html).
Qualification: The liquid phase doesn't exist in thermodynamic equilibrium if the pressure is too low. If you suddenly inject the liquid into a low pressure environment, it is out of the thermodynamic equilibrium and will persist as liquid for some time---how long depends on the conditions---which for Mars, yours truly doesn't know.
The figure above (
local link / general link: phase_diagram_water.html) suggests that at Martian surface pressure (about 0.06 of Earth's pressure or about 0.600 kPa) that liquid water could exist if the temperature rose above 0° C.
It may be that on the warmest days on the Martian equator or in the Martian summer hemisphere in the deepest valleys that liquid water could persist---but there's no evidence that there is any under such conditions.
The Composition of the Martian Atmosphere:
The Martian atmosphere is CO_2 dominated like the atmosphere of Venus---but there is VERY LITTLE greenhouse effect because of the low density.
The table below shows the composition of the Martian atmosphere.
There is some water vapor which can condense in the air to make white water ice clouds as we maybe??? saw with the Olympus Mons picture above.
CO_2 ice clouds can make clouds at high altitudes too (FMW-200--201).
Probably early on, Mars had a significant CO_2 and water vapor atmosphere from volcanic outgassing like the early Earth and early Venus are thought to have had.
In fact, Mars may have had several intervals of such an atmosphere as we will discuss below in the section Water on Mars.
But because of the low gravity of Mars and probably solar wind in the absence of a global magnetic field, much of this early thick Martian atmosphere has undergone atmospheric escape over the gigayears.
The fractionation of the argon isotopes in the Martian atmosphere provides direct evidence of atmospheric escape. See the figure below.
Probably the molecules of CO_2 and H_2O were broken up at high altitudes and the C and H escaped mostly.????
Credit/Permission: NASA/Jet Propulsion Laboratory (JPL)-Caltech, 2013 (uploaded to Wikipedia by User:Drbogdan, 2013) / Public domain.
Image link: Wikipedia: File:PIA16818-MarsCuriosityRover-Argon-AtmosphericLoss.jpg.
O_2 on Mars:
Any oxygen in the form of O_2 did NOT stay, and there may never have been much build of it in the Martian atmosphere if Mars never had biological production.
The reddish color of much of the soil and rocks of the Martian surface is due to to iron(III) oxide.
Mars really is the Red Planet or reddish planet viewed from most of the Martian surface. This is illustrated in the figure below.
The Colors of Mars:
Actually Mars's colors are tricky.
From Earth, it looks reddish-orange with the naked eye??? (PF-118).
Telescopic observers see a mix of colors including red and orange and dark.
Close up orbiter images often show the bright areas as more yellowy-browny than red.
The reddish surface, as discussed above, is because of the high amount of iron(III) oxide (Fe_2O_4) (see Wikipedia: Martian surface: Albedo and Color).
The dark or blackish regions (like Syrtis Major) are because of a high amount of iron(II) oxide or ferrous oxide (FeO) Wikipedia: Martian surface: Albedo and Color.
Then there is the problem of the Martian sky color.
Images typically use filters to bring out features, NOT show true color as seen by the human eye.
The Martian sky color does vary, but the daytime color seems to be mainly a bright yellow-brown. See the figure below.
Near sunrise and sunset the color is mainly rose, but just near the Sun the color is blue.
Climate on Mars:
Because of its low pressure and hence lack of liquid water, Mars today is essentially a cold desert.
Mars does have seasons because of its axial tilt (and secondarily its orbital eccentricity) and weather.
Seasons on Mars:
The Martian seasons somewhat mimic Earth's and are most noted through the variations in the Martian polar ice caps. See the figure below.
The Martian polar caps are coated in a layer CO_2 ice (dry ice) over a much thicker layer of water ice.
In a Martian summer hemisphere, some of the CO_2 evaporates (properly speaking sublimates) and goes into the Martian atmosphere. So the summer Martian polar cap shrinks.
At the same time, the winter Martian polar cap in the other Martian hemisphere (the winter hemisphere) grows as CO_2 from the Martian atmosphere condences (properly speaking deposits) on it.
So the north polar cap and south polar cap alternatively grow and shrink with the Martian seasons.
The figure below shows a simplified version of the behavior of Martian volatiles which implies the seasonal cycle of the Martian polar caps.
Besides the Martian polar cap, CO_2 gas condences at other places on Mars too as a CO_2 frost as illustrated in the figure below.
Weather on Mars:
The weather on Mars is most notable through the winds blowing Martian dust and dust storms.
These dust storms can be global, but I've never seen an image where it was easy for me to notice that a dust storm was happening---it must take a trained eye.
A smaller Martian dust phenomena are the Martian dust devils.
Dust devils are little whirlwinds which we have on Earth too including in Arizona and Nevada---there was one in my parking lot once. See the figure below.
See Martian dust devils in Mars videos below (local link / general link: mars_videos.html):
Wind erosion and wind building of dunes is a common feature of Mars too---but we leave it at that for now---except for the image of Martian sand dune shown in the figure below.
The Martian atmospheric dust is reddish in color (or yellowish-brown, but it's in the eye of the beholder) and its magnetic component is due to magnetite (iron oxide Fe_3O_4) (a black iron oxide) (see Kerr, R. A. 2005, Science, 308, apr08, 192).
Magnetite is NOT formed in water and that it is in the Martian dust suggests much of Mars has been mostly dry for a long time.
Martian dust and other evidence is pointing to Mars having had a dry surface for maybe 3.7 Gyr, except perhaps for brief episodes (see Kerr, R. A. 2005, Science, 308, apr08, 192).

Group Activity:

Form groups of 2 or 3---NOT more---and tackle Homework 14 problems 9--14 on Martian geology, etc.

Discuss each problem and come to a group answer.

Oh, 5--10 minutes.

See Solutions 14.

The winners get chocolates.

See Solutions 4.

Water on Mars

The subject of water on Mars has been heavily investigated.

There is more than yours truly can say.

So we skim?

Liquid Water on the Surface of Mars:
There is significant Water on Mars---but there is no continuous liquid water on the surface.
And it is liquid water that is needed for life as we know it and liquid water erosion.
A Catalog of Water on Mars:
Let's do a little catalog of water and possible water on Mars.
1. In the atmosphere, there is about 0.03 % water vapor by number as noted above.
  This is much lower than the 0.1--2.8 % range on Earth (CW-296), and recall Earth's atmosphere is about 100 times denser than Mars's atmosphere.
2. There is water in the soil in the form of isolated molecules embedded in the crystal structure of rock grains. This was discovered by the Viking landers (HI-188).
3. There is substantial water ice. The polar caps as noted above probably have 1 to 3 km of water ice in places (HI-187).
4. Although NOT proven absolutely, it is considered highly likely that there is water ice in the soils at higher latitudes (HI-188).
5. At deep enough depths where pressure and temperature are sufficiently high, there well may be liquid water in aquifers (HI-195).
6. There is solid evidence now for Martian glaciers on that formed within millions to tens of millions of years ago as one can tell by cratering on them (Kerr, R. A. 2005, Science, 308, apr08, 192). One is at the base of Olympus Mons for example.
  The glaciers are rock-and/or-dust-coated, and so hidden. It has taken detailed imaging by the NASA Mars Global Surveyor and the ESA Mars Express Orbiter (first operational 2004) and radar mapping by the NASA Mars Reconnaissance Orbiter to find their signatures.
  They are NOT like most terrestrial glaciers, but rather like those in the McMurdo dry valleys in Antartica---they ooze over things rather than slide on a wet layer.
  Probably the Martian glaciers formed from straight vapor to solid transitions. There was no liquid water involved.
  They probably are NOT growing now, but are relics of a time when Mars's axial tilt was larger and more water could be sublimated from the polar caps and spread to other regions. The axial tilt seems to vary because of some effect???.
Martian Gullies:
As noted in the section The Martian Atmosphere, liquid water CANNOT exist now on the surface for any length of time because of the low pressure and low temperature.
It can exist for short times before it evaporates or freezes.
But does it?
It's very uncertain.
In the last few years the Mars Global Surveyor's Mars Orbiter Camera (MOC) has been finding evidence of fairly recent gullies on Mars.
The figure below shows Martian gullies and gives some description.
Where is the liquid water---if there is any on the surface ever---coming from?
Well you don't need to go far down into the ground before the pressure rises high enough for liquid water to exist.
Since we argued above that Mars has still modest internal heat, at some depth beneath the ground it is probably warm enough as well as high enough in pressure for liquid water.
Thus, it's possible, as noted above, that underground water reservoirs (i.e., aquifers) exist today (HI-195).
Subsurface events may sometimes squeeze liquid water to the surface where it flows out of canyon or crater walls and forms gullies in the brief time before it evaporates or freezes.
BUT a recent theory says the "gullies" may be slumping of the powdery surface, and thus a purely dry phenomenon. See Nature: Shinbrot et al. Just when you think you understand something, some crazy scientist comes along and ...
There has been a nugget of evidence from chemical deposits for the presence of liquid water on maybe a yearly or millennial basis, but it's NOT very exciting (Nature: Liquid Water Found on Mars, But It's Still a Hard Road for Life, 2010oct29). In any case, it's NOT on the surface, but below CO_2 ice layers.
A Wetter Mars in the Past:
Was Mars wetter in the past? Did it have continuous surface water?
Well ever since the Viking orbiters of the 1970s these questions have been hotly debated.
Images from the Viking orbiters showed many striking channel features---dry of course.
Some of them may be flood channels due to a sudden melting of water ice and release of liquid water by a volcanic eruption or an impactor.
But some channels look very much like rain-fed runoff channels such as we find on Earth.
For rain and surface liquid water to exist, Mars's atmosphere would have had to have been substantially denser and warmer than today.
There is evidence for this as discussed in the figure below.
There are in fact a huge number of gully images and other images from the Mars Global Surveyor's Orbiter Camera (MOC). See Malin Space Systems: Mars Global Surveyor Mars Orbiter Camera Captioned Image Releases.
But was there ever continuous running water on Mars?
Even before the Mars Exploration Rovers (Spirit (2004--2010) and Opportunity (2004--2018)), there was confirming evidence that Mars had continuous running water for some significant periods of time: years? decades? centuries? megayears? I havn't found any estimates yet.
The confirming evidence came from the Mars Global Surveyor and is shown in the figure below.
So yes, Mars, had at least rivers and bodies of water of some size.
The Mars Ocean Hypothesis:
Did Mars ever have oceans?
This topic has been hotly debated for years with different groups coming to different conclusions based on the studies of basin shapes and rock formations.
The 800 lb gorilla of an idea is the Mars Ocean Hypothesis that posits that virtually the whole blue region in the topographic map in the figure below was an ocean about 3.8 gigayears ago.
We won't go into the full evidence for the
Mars Ocean Hypothesis---cause I've NOT had time to bone up on it.
However, the Mars Exploration Rovers (Spirit (2004--2010) and Opportunity (2004--2018)) have added some evidence in favor of oceans and have proven that there were at least desert basin salt lakes.
Some images illustrating the mission of the Mars Exploration Rovers are shown in the three figures below.
And the evidence from the Mars Exploration Rovers for oceans and at least desert basin salt lakes is shown in the dramatic figure below. These kind of images just make geologists go wild.
It seems that Opportunity in the Meridiani Planum was the lucky landing site with lots of water evidence. Spirit in Crater Gusev has come up dry it seems.
The Curiosity Mars rover also found evidence of water. See the figure below.
There are lots more Mars Exploration Rover images and results, but that's enough for now.
The site Views of the Solar System: Mars offers a good selection of rover and other images of Mars.
See also Mars videos below (local link / general link: mars_videos.html):

Group Activity:

Form groups of 2 or 3---NOT more---and tackle Homework 14 problems 9--14 on Martian geology etc.

Discuss each problem and come to a group answer.

Oh, 5--10 minutes.

See Solutions 14.

The winners get chocolates.

Life on Mars

Is there or has there ever been life as we know it on Mars (i.e., Martian life)?

Life as we don't know it is harder to debate. There is no evidence of it existing at all. But who knows.

Martian Life Possibly:
From the discussion above in the section Water on Mars, it seems that Mars was sometimes warmish and wet.
In other respects, early Mars was also probably like early Earth with a CO_2 and water vapor atmosphere.
We think that microbial life first appeared on Earth 3.465 Gyr ago at least and perhaps 4.28 Gyr ago (see Wikipedia: Earliest known life forms; Wikipedia: Abiogenesis). But we don't know how exactly it arose. But somehow:
If the 3.465 Gyr value is correct, it didn't take long---geologically speaking---for life to arise after the HEAVY BOMBARDMENT was over.
Thus, it is possible that microbial life arose quickly on early and sometimes warm and wet Mars.
Transpermia:
It is also possible that life originated once on either Earth or Mars and was transferred to the other by impact events. Or perhaps some more complex multiple origins and transfers happened.
The process of life spreading through impactor-ejected rock has been called transpermia. See P.C.W. Davies: How bio-friendly is the universe.
The cartoon in the figure below illustrates transpermia in action.
An impactor throws rocks containing microbes into space.
The microbes stay in some kind of suspended animation and then re-animate if the rock impacts on a hospitable planet.
The rocks have to stay below about 100° C and be relatively unshocked, but calculations show that some material will stay this cool and unshocked both through ejection and impact.
Significant transfers of materials between planets do happen this way and it is believed that some kinds of microbial life would survive the journey.
Biological contamination back and forth between Earth and Mars may even have happened repeatedly in the early solar system.
Reading about transpermia has given me more confidence in the possibility of Martian life---but Martian life affected by transpermia will probably NOT be independent of Earth life.
Famous, But Largely Dismissed, Evidence for Life on Mars:
And there is some famous, but largely dismissed, evidence for microbial life on early Mars.
The evidence is meteorite Allan Hills 84001 show and described in the figure below.
Life on Mars Today?
Could life still exist on Mars today?
Well there can't be life as we know it on the surface---unless it can carry its own liquid water supply and shield itself from ultraviolet (UV) light.
Also the Viking landers did tests for organic compounds in Martian topsoil which would have been some evidence for Martian life and found nothing (HI-157).
But that evidence has been recently disputed. A re-analysis of Viking lander tests suggests that they did find organic compounds after all (Washington Post: New Mars tests find possible life ingredients, 2010sep03). It's all very uncertain though.
We need fresh tests for organic compounds in Martian soil and remarkably none of the landers since Viking have done those tests.
So far the Mars Exploration Rovers and all other probes have found no signs of life either.
But microbial life could exist in the aquifers that probably exist below the surface or in other moist subsurface environments? Recall it seems likely that aquifers exist today (HI-195).
Earth provides us with evidence by analogy.
Since circa 1990, we have known microbial life on Earth reaching down to 3 km or more below the surface. This deep life has been collectively called the deep biosphere.
Additionally, microbial life discovered 3.5 km below the Antarctic ice. This life has been isolated from the surface for more than 1 Myr.
It has also been claimed that some terrestrial microbes can be dormant for 100 Myr or more (HI-157). So its possible that Martian life could endure long periods of poor conditions and resurrect in better times.
So it seems possible that something like Earth microbes could live on Mars below the surface today.
But do they?
Probes for Martian Life:
So we have both possibilities: life in the past on Mars and life today.
To find out we need probes.
The Mars Exploration Rovers (Spirit and Opportunity first operational 2004) did NOT find evidence of life: e.g., fossil remains.
The European Space Agency's (ESA's) Mars Express Orbiter (first operational 2003) found a bit of evidence consistent with life as we know it.
Mars Express Orbiter is shown in the figure below.
The evidence?
Mars Express Orbiter has spectroscopically confirmed the existence of methane (CH_4) in the Martian atmosphere (see ESA Press Release: Mars Express confirms methane in the Martian atmosphere; Wikipedia: Atmosphere of Mars: Methane).
The amount detected is tiny: 0.01 ppm = 10 ppb.
Methane is a main component of natural gas.
But methane CANNOT last in the Martian atmosphere for more than a few centuries because it breaks up to form CO_2 and water using oxygen present in some form.
Therefore methane must be continually produced on Mars.
Volcanic or hydro-thermal activity on Mars could produce methane.
If this is the source, then it confirms our believe of continuing primordial-radiogenic heat geology.
On the other hand, methane can be produced biologically (e.g., by fermentation).
So the possibility of subsurface microbial life is raised.
Further studies of the abundance and distribution of methane might yield more insight.
Earth Invades!
In the search for Martian life, we may get lucky.
The Mars Exploration Rovers (2003--2018) (Spirit (2004--2010) and Opportunity (2004--2018)), did NOT find fossils---but some new rover could tomorrow---and tomorrow is just a daaaay awaaaay.
But yours truly suspects that the search will have to go on for a long time.
It would be a pity if there were no Martians (see the figure below: local link / general link: alien_prototype_are_here.html) to observe our slow-motion invasion.
It is The War of the Worlds (1898)---first, we send our monster robots, then ...